1. Field of the Invention
This invention relates to methods and apparatus for electrostatically coating a workpiece with a layer of powder such as paint powder, other synthetic resin powder and porcelain enamel frit, and in particular, to methods and apparatus particularly suitable for forming a thick powder layer at a high speed over a workpiece having a high electrical resistance such as a glass bottle.
2. Description of the Prior Art
In recent years, it has been desired to form a film of synthetic resin over the outer surface of glass bottles for use as containers for carbonated drinks such as Coca Cola (Trade Mark) and a certain carbonated cider-like drink in order to prevent accidents due to the failure or rupture of the bottles resulting from movement during shipping or an increase in the internal pressure when exposed to the sun. The desirability of this is increasing due to an increase in the size of the glass bottles.
One method for forming such a coating of synthetic resin is the electrostatic powder coating method. However, when this method is applied to a glass bottle, which is electrically insulative, the bottle must typically be heated to a temperature above the melting point of the synthetic resin powder. Since the glass bottle is heated during the coating process to above the melting point of the powder, it is almost impossible to remove the powder once placed on the bottle. Therefore, it is typical to use a gas jet for masking the vicinity of the mouth of the bottle in order not to coat the mouth of the bottle.
However, it is almost impossible to accomplish perfect masking for the powder in practical manufacturing lines, and when the mouth portion of the bottle on which a cap is to be placed is coated with the synthetic resin powder, serious problems such as imperfection in capping and decapping and ingress into the interior of the bottle of separated synthetic resin film flakes result. Also since the grippers of the conveyor for holding the glass bottles are heated due to thermal conduction from the pre-heated glass bottles, the synthetic resin powder deposited on the grippers is also a heated melt and accumulated thereon. In order to remove this accumulated resin, the conveyor must be periodically stopped, making continuous operation impossible. Also, the resinous film on the bottle neck has an uneven edge of a reduced uneven thickness, causing the film to be easily separated and broken at the upper edge during the washing, carrying and handling of the bottle. This results in a reduced life of the glass bottle, which is a critical concern for returnable glass bottles. Also, although it is desirable to form a straight neat edge of the resin film on the bottle for increasing the commercial value of the bottle, it is extremely difficult with the above described electrostatic coating method using preheated bottles to form a satisfactorily neat film edge.
In order to eliminate many of the above mentioned disadvantages relating to the prior art method for manufacturing resin coated bottles wherein a preheated glass bottle is coated with a layer of synthetic resin powder formed using a conventional electrostatic powder coating gun and then baked to form a hard coating, it is conceivable first to form a powder layer of a predetermined thickness on a bottle at room temperature, removing the resinous powder deposited on the undesired portion of the bottle by a suitable means, and then to heat the glass bottle on which a resinous powder layer of just enough thickness is formed on the desired portion only to thereby manufacture resin coated glass bottles. However, it is impossible to form a powder layer of desired thickness (Typically, a thickness of from 150.mu. to 400.mu. is required for the coating film after it has been baked) on a glass bottle at room temperature at the high speed required in the industrial manufacturing process with a conventional electrostatic powder coating apparatus for the reasons which will be described in detail below.
A typical conventional electrostatic powder coating gun achieves electrostatic powder coating by positioning a corona discharge electrode such as a corona pin or a corona edge within or in the vicinity of a nozzle for ejecting synthetic resin powder suspended in a gas, applying a high voltage across the electrode and a workpiece which is positioned in opposingly spaced relationship with the electrode to charge the suspended powder or particles ejected from the nozzle with the ion current which is generated from the corona discharge electrode and which is directed to the workpiece, and at the same time utilizing the electric field directed from the corona discharge electrode to the workpiece. However, when a workpiece having a high electrical resistance such as a glass bottle is coated with such an electrostatic coating gun, electric charge accumulates on the surface of the workpiece to increase the electrical potential thereof due to the high ion current flowing the corona discharge electrode to the workpiece. Therefore, the potential difference between the corona discharge electrode and the workpiece decreases to eventually reduce the electric field therebetween, and the corona discharge is also reduced, making it impossible to maintain the continuing discharge necessary for charging the powder. Therefore, with the just mentioned conventional electrostatic powder coating gun, it is completely impossible to form a synthetic resin powder layer of a desired thickness on the glass bottle with high reliability at room temperature.
U.S. Pat. No. 3,976,031 discloses a discharge coating apparatus wherein a workpiece and a silent discharge plate electrode are disposed in an opposing spaced apart relationship, a powder coating material being supplied therebetween, with the apparatus including means for applying an a.c. voltage for establishing silent discharge from the silent discharge plate electrode, and means for applying an electric voltage across the workpiece and the silent discharge plate electrode. When this discharge coating apparatus is used to coat a glass bottle, it is normally impossible to form a satisfactory coating film due to the decreased potential difference between the glass bottle which is the workpiece and the silent discharge plate electrode because the ion current flowing from the silent discharge plate electrode to the glass bottle is high. Although with this discharge coating apparatus the ion current flowing from the silent discharge electrode to the workpiece can be decreased to a relatively small amount, thereby enabling a relatively thick film of coating powder to be formed on the glass bottle at room temperature, this decreases the speed of the production line, rendering this prior art discharge coating apparatus to be inadequate for use in present bottle manufacturing plants.
In Japanese Laid-Open Patent Application No. 50-22839, laid open on Mar. 11, 1975 and corresponding to U.S. Pat. No. 3,991,710 granted to Gouridne et al. on Nov. 16, 1976 an electrogasdynamic powder coating apparatus is disclosed. The coating apparatus disclosed therein comprises a charged particle formation chamber and a deposit chamber disposed downstream from the charged particle formation chamber, a high voltage being applied to the inner wall thereof. Through these chambers, a cloud of charged particles and a workpiece are successively moved, to coat the workpiece with the particles solely by a space charge electric field established by the charged particle in the charged particle formation chamber, and to further coat the workpiece with the charged particles remaining after the previous coating process by utilizing an electric field formed by the high voltage applied to the inner wall of the deposit chamber in the deposit chamber. However, for the reasons which will be described below in detail, it is extremely difficult to use the electrogasdynamic powder coating apparatus disclosed in the above cited application on a large industrial scale in coating glass bottles at room temperature with powder particles for the purpose of preventing breakage of the bottles.
The synthetic resin coating film required for the purpose of preventing breakage of bottles must be very thick compared to ordinary electrostatic powder coating film as previously described. In order to form such a thick coating film, the main grain size of the synthetic resin powder used must be within the range of between 60.mu. and 150.mu.. With the method disclosed in the aforementioned U.S. Pat. No. 3,991,710 these powder particles fall out within the coating apparatus at a high rate, making it difficult to maintain a satisfactory coating efficiency and production line speed. In particular, within the charged particle formation chamber in which powder coating is achieved by the space charge electric field alone, the strength of the space charge electric field decreases at a very high rate according to the distance from the workpiece because there is no external electric field present, lowering the coating efficiency. This is aggravated by the high fall out rate of the particle due to gravity. Also, as for charging within the charged particle formation chamber, it is extremely difficult to always maintain a perfect charge on the particles with the production system such as the system for manufacturing glass bottles required to be operated for very long continuous periods. From this point of view it is difficult to realize a satisfactory production line speed and a satisfactory coating efficiency.
When a voltage high enough to drive the charged particles toward the workpiece is applied to the inner wall of the deposit chamber, powder particles charged in the opposite polarity, which are formed to some extent in the charged particle formation chamber, deposit on the surface of the inner wall of the deposit chamber. The electric charge thus accumulated in the deposited powder layer brings simultaneous backdischarge within the deposited powder layer, causing a heavy current to flow from the inner wall to the workpiece. This electric current destroys the powder layer formed on the workpiece within the charged particle formation chamber and separates the powder from the workpiece. Therefore this method is practically impossible to use in coating glass bottles with synthetic resinous powder. In order to avoid this phenomenon, which is an obstacle in coating and which is derived from the backdischarge in the interior of the powder layer charged in the reverse polarity and accumulated on the inner wall surface of the deposit chamber, the voltage applied across the inner wall must be extremely low. However, with the extremely low voltage, the electric field directed from the inner wall to the workpiece is not strong enough to drive the charged particles toward the workpiece, resulting in severe difficulty in utilizing the apparatus for coating glass bottles at room temperature with synthetic resin powder for the purpose of preventing breakage of the bottles. Also, for the above reasons, the powder which has not contributed to coating of the workpiece rapidly accumulates in the deposit chamber making it impossible to operate the apparatus continuously.
In summary, the method as disclosed in above cited U.S. Pat. No. 3,991,710 although effective for powder particles of mean grain size of 50.mu. or less and very easily charged in a single polarity, is not satisfactory for coating the workpiece with a thick layer of powder particles of relatively large grain size at a very high speed.